The diagnosis of depression is clinical, based on an assessment, and quite straightforward. A patient meets the ICD criteria for a depressive episode when, for at least two weeks, he/she experiences at least two of the three core symptoms (consistent sad mood; decrease in interest and pleasure; and reduction in energy or tiredness after minimum effort) and at least two of the seven secondary symptoms  (reduced concentration and attention; feeling guilty and worthless; reduced self-esteem and self-confidence; appetite and weight variations; sleep disturbances; pessimistic and ruminative ideas; suicidal ideation and suicidal acts).

The diagnosis is syndromic and relies on three characteristics: symptoms have to be recognised and duration must be clearly longer than 2 weeks and social, interpersonal and occupational functioning must be negatively impacted.
The ICD-10 covers depression under the following codes: F31, bipolar disorder; F32, depressive episode; F33, recurrent depressive disorder; F34.1, dysthymia; and F06.3, mood disorder due to organic conditions.